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Re: extinction (long)



First a few thoughts about ?world-wide wildfires?. World-wide does not mean ?the whole world?. All studies of ejecta distribution indicate that the high-energy debris would be very unevenly distributed. There would under almost any circumstances be a heavy concentration antipodal to Chicxulub (in India or thereabouts) but otherwise the distribution would depend strongly on the direction of the impact and the rotation of the Earth. It has even been suggested that the distribution of wildfires might be useful for determining the direction of the impactor.

Also studies on the effects of hydrogen bombs (fortunately mostly theoretical) show that the presence or absence of cloud-cover, the wetness of the vegetation and the presence of snow-cover greatly influences the energy required for actual ignition.

It should also be noted that sites relatively near the impact site (such as Colorado) could only have been hit by relatively low-energy debris (an object moving at 3 kms-1 has less than 15% of the energy of one moving at 8 kms-1). This is because only objects moving in strongly curved paths can hit close to the primary impact site.
Admittedly high-energy objects that had completed one or more orbits might impact practically anywhere, but by this time (at least a couple of hours after the impact), the flux would have been to low for ignition. Any pieces moving at more than 11 kms-1 would of course be ejected into orbits around the sun (actually closer to 12 kms-1 in some cases due to the rotation of the Earth). Many of these, plus those ejected into more-or-less stable orbits around the Earth will ultimately fall back. There must have been lots of shooting-stars for a LONG time after Chicxulub.


It is not at all clear (and indeed unlikely) that soot from even very large wildfires would be distributed world-wide, in contrast to particulate matter from the impact. The latter was carried to the top of the atmosphere entrained in the rising fireball (which was large enough to ?punch through? the atmosphere) or ballistically distributed into the top of the atmosphere. The larger pieces would come down fast while the fine material would remain in the stratosphere for a long time and be fairly evenly distributed across the world by high-altitude winds.
The soot on the other hand is produced at ground level and carried up into the atmosphere by the convection produced by the heat of the fire. This means that it will under normal circumstances stay within the troposphere and be washed out by precipitation within a fairly short period. It will also mostly remain in the hemisphere where it was formed, since there is relatively little tropospheric exchange across the tropical zone. This is supported by experience. Volcanic debris from explosive eruptions, which is lofted to the stratosphere, spreads world-wide. Examples: Pinatubo, Krakatoa, Tamboro. Smoke, even from very large fires, does not. Examples: very large forest-fires in Indonesia a few years ago, oil-fires in Kuwait 1991, large-scale fire-raids in 1943-45. It has been suggested that fire-storms might cause strong enough convection to carry smoke into the stratosphere (?nuclear winter?), but I have never heard of any case where this has actually happened.
Gaseous products of the fires (e. g. CO2, CO, NOx and a large range of hydrocarbons many of which would be highly toxic and/or mutagenic) would remain in the atmosphere for a longer time, and these might well have a world-wide effect.


It is true that flood basalt eruptions are ?quiet?, but that does not mean that they do not have large-scale environmental effects. The Lakagigar eruption of 1783 (the only historical (small) flood basalt eruption) had strong effects on weather in the northern hemisphere, presumably through sulphuric acid emissions. The strange haziness and cold weather was noted by Benjamin Franklin who was then living in France and even correctly attributed by him to the eruptions on Iceland, though he thought that it was Mount Hekla that was erupting. The local effects on Iceland were devastating, mostly because of a bluish sulfurous haze that covered most or all of Iceland during the summer of 1783 stunting vegetation and poisoning cattle. The resulting famine is remembered as móduhardhindin ?the haze famine? and killed an estimated third of the population. It has even been suggested that the bad harvests caused by the Lakagigar eruption in Europe may have been a contributing factor in causing the French revolution. Incidentally it is thought that the cattle were mostly killed by fluorine poisoning, so it would be interesting to know whether there is a world-wide ?fluorine peak? in 1783. Unfortunately it is difficult to think of two elements with more different chemical characteristics than Ir and F.
In this context it is interesting to note that since dinosaurs (and many other organisms) occur in the intertrappan deposits in India at least the early stages of the Deccan volcanism were survivable, even on the Greater India plate itself. Since this was situated on top of the Reunion hotspot at the time, extinction and re-colonization (all within the late Maastrichtian) is unlikely. It would take a pretty remarkable ramp-up of the volcanism after the deposition of the inter-trappan beds to cause mass extinction even in the antipodes. Is there any evidence of this?


As for the discussion of the Chicxulub drilling that started this thread I fail to see how a single borehole would make it possible to decide how large the crater is. Actually the YAX-1 hole is 60-70 km from the center of the crater and so would be well OUTSIDE the crater rim if Chicxulub was only 100 km in diameter.
The melt sheet may be thinner than expected (IF the boring really went completely through the melt sheet and the (mostly) unmelted rock encountered in the lower part is not just megablocks from the collapsing crater wall), but then how many melt sheets have been drilled through before? Zero. So the expected thickness is entirely based on theoretical models. How many times have these been validated previously by comparison with an actual large crater? Also zero. Further the cenote ring is definitely there and it has a diameter of some 200 km (have a look at http://www.geotimes.org/apr03/NN_chicx.html). Is there any other case known where an impact crater manifests itself in a precisely circular structure at twice the diameter of the crater itself?


Actually the figure depends on how one defines the crater diameter. If one uses the outer limit of the deep crater structure (ring faults) then Chicxulub is probably 160-170 km across. This is probably the best measure when comparing it with deeply eroded terran craters such as Sudbury or Vredevort. If it is measured across the crater rim the diameter is probably 200-220 km. This is more relevant when comparing it with lunar and Martian craters, which we see from above and which are usually fairly well preserved. In either case it?s a goddam big crater. There are very few larger ones of post-Late Heavy Bombardment age on either the Moon or Mars.

A few thoughts on environmental effects of a large impact that have not been much noted:

If the total amount of vaporized seawater was 10^12 tons (the figure of 3-7 x 10^11 tons seems definitely on the low side) this would be equivalent to about 4 mm of precipitation if spread evenly over the northern hemisphere. Certainly a very significant quantity but not catastrophic by itself. However much of the seawater vaporized by the impact would have been carried into the stratosphere together with massive amounts (megatons) of chlorine (also iodine, bromine &c). This must have had drastic effects on the ozone layer. Incidentally, since most of the chlorine in seawater is chloride of sodium, the skies at dawn and dusk would probably have been yellow for quite a while after the impact. Note that vaporization would also have gone on for a while after the impact. Once the fireball had dispersed seawater would pour into the crater to be boiled away until the temperature of the interior had been quenched to below 100 centigrade. This water vapour would stay in the troposphere though, and rain out fairly quickly.

The tsunamis from the impact would probably have been high enough to break at the edge of the continental shelf, at least in the Caribbean and parts of the Atlantic. This would have greatly reduced the effects onshore, except where the shelves were narrow, but on the other hand it would have violently disturbed vast amounts of sediment that would normally never be affected by wave action. Large-scale shelf collapse and turbidity-currents are likely and a major ?belch? of methane from methane hydrates is quite possible. The magnitude 10-13 earthquake would have had similar effects, particularly in the Caribbean area.

Finally I recommend visiting:

http://www.lpi.usra.edu/meetings/lpsc2003/lpsc2003.download.html

which has a lot of interesting information on the results of the YAX-1 borehole, written by impact researchers with no paleontological axes to grind.


Tommy Tyrberg